1. Field of the Invention
The invention relates generally to television display apparatuses having two image areas on a screen, and more particularly, to a television display apparatus having picture-in-picture display function for displaying television signal from one signal source in a large image area on a screen of the display apparatus, while displaying television signal from the other signal source in a small image area which is a portion within the large image area on the screen of the display apparatus, in accordance with video signals included in the generated television signals from two signal sources, and a method of picture-in-picture display.
2. Description of the Background Art
Various types of television display apparatuses having a picture-in-picture display function are already proposed. For example, Japanese Patent Publication No. 59-37913 discloses a television receiver receiving television signals of the NTSC system transmitted from two broadcasting stations and having a picture-in-picture display function for displaying a picture of one broadcasting station's television signal as a scale-reduced picture in an area within the display area of the picture displayed by the other broadcasting station's television signal (hereinafter referred to as PinPTV receiver).
In summary, the PinPTV receiver of the above mentioned prior art comprises two field memories for storing the first field and the second field of television signal of the small image area, a first field detecting circuit for detecting whether the television signal of the large image area is of the first field or the second field, a second field detecting circuit for detecting whether the television signals of the small image areas is of the first field or the second field, and a memory controller for delaying the reading of the television signal of the small image area by one horizontal scanning period when the television signal of the large image area is of the second field and the television signal of the small image area is of the first field.
The object of the above mentioned prior art is to reverse the interlace relation when the television signal of the large image area is of the second field and the television signal of the small image area is of the first field to obtain an appropriate small image area. In the case of reading the first field of the small image area when the video signal of the large image area is the second field, the memory controller delays the first field of the small image area by one horizontal scanning period to prevent the reverse of the interlace relation, thereby to achieve an appropriate picture.
In recent years, a high definition television system other than the standard television system is proposed as the television system. In such a case, displaying in combination an image area of television signal under the standard television system within one area of a large image area of television signal under the high definition television system is considered, for example.
A method for band-compressing the television signals of the above mentioned high definition television system (1125 lines, 60 fields) and transmitting the same has already been proposed. The proposed method includes multiplex sub-sampling transmission method employing offset sub-sampling between fields and frames. A system called MUSE (Multiple Sub-Nyquist Sampling Encoding) is known as one type of multiplex sub-sampling transmission system.
The decoder of such a MUSE system will A/D convert the transmission signal for digital signal processing and restore it to the original television signal of the high definition system, to be used as an analog signal by D/A conversion afterwards. In this case, the clock signal used in A/D conversion and digital signal processing is created by frequency dividing the original oscillating signal of 97.2 MHz in phase with the synchronizing signal included in the television signal.
The current standard television system, the NTSC system for example, generates a color subcarrier of 3.58 MHz in synchronization with the color burst signal included in the horizontal blanking period in extracting the necessary color information from a carrier chrominance signal.
However, the standards of a standard television system, for example, the NTSC system and the high definition television system differ from each other. For example, the aspect ratio of the screen is 4:3 and the scanning lines 525 lines/frame in the NTSC system, while the high definition television system has an aspect ratio of 16:9 and 1125 lines/frame of scanning lines. The frequencies of the horizontal synchronizing signal and the vertical synchronizing signal also differ between the NTSC system and the high definition television system. Therefore, it is not easy to display a small image area of television signal under a standard television system within one portion of a large image area of television system under the high definition television system.